1. Field of the Invention
This invention relates to a method and apparatus for evaporating a deposition material, such as a magnetic material, and depositing a thin film of the deposition material on a substrate of a tape, a disk, or the like, in a vacuum. This invention particularly relates to a vacuum evaporation method and a vacuum evaporation apparatus wherein a deposition material is evaporated with an electron beam.
2. Description of the Prior Art
As media for recording and reproducing magnetic information, coated types of magnetic recording media have heretofore been used widely. The coated type of magnetic recording media comprise a non-magnetic substrate and a magnetic layer overlaid on the non-magnetic substrate. The magnetic layer is formed by applying a magnetic coating composition, which contains magnetic grains and an organic binder, or the like, to the non-magnetic substrate and then drying it.
Nowadays there is a strong demand for magnetic recording media on which magnetic information can be recorded at high densities. This demand cannot be satisfied with the conventional coated types of magnetic recording media. For example, in a Hi-8 system, magnetic information is recorded at a high density with wavelengths shorter than 0.5 .mu.m. For such a system, thin metal film types of magnetic recording media are substantially suitable and have been used in practice. The thin metal film types of magnetic recording media comprise a non-magnetic substrate and a thin ferromagnetic metal film overlaid on the non-magnetic substrate. The thin metal film types of magnetic recording media are advantageous in that they have a high magnetic energy level and the thickness of the magnetic layer can be kept thin.
Thin metal films can be formed with wet processes, such as electrolytic plating and electroless plating processes. Thin metal films can also be formed with dry processes, such as vacuum evaporation, ion plating, sputtering, and chemical vapor deposition (CVD) processes. From the point of view of the rate, with which thin metal films are formed, and the productivity, vacuum evaporation processes are most advantageous among the above-enumerated processes.
As examples of thin metal film types of magnetic recording media, vapor deposition tapes are utilized. Vacuum evaporation processes are also utilized to make vapor deposition tapes. In the vacuum evaporation processes, electron beams are employed widely such that metals, which have high melting points and primarily contain Co, may be evaporated quickly. Particularly, among the vacuum evaporation processes utilizing electron beams, oblique incidence vacuum evaporation processes are employed such that necessary magnetic characteristics, e.g. the coercive force (Hc) and the squareness ratio (SQ), may be obtained. With the oblique incidence vacuum evaporation processes, only part of a stream of vapor is deposited on a substrate.
FIG. 12 is a schematic view showing a conventional vacuum evaporation process. A deposition material 2 is contained in a refractory crucible 1. The deposition material 2 is heated, molten, and evaporated by an electron beam 3. The resulting stream of vapor 4 of the deposition material 4 impinges upon a substrate (not shown), and a thin film of the deposition material 4 is thereby formed on the substrate.
FIG. 13 is a perspective view showing the crucible 1. In general, the crucible 1 has a large width, which width is taken in the width direction of a substrate, and the electron beam 3 scans the deposition material 2 along the width direction of the crucible 1 such that the stream of vapor 4 may continuously impinge upon a wide substrate.
As descried in, for example, "Vacuum Handbook", Nihon Shinku Gijutsu K.K., p. 231, in cases where the source of the stream of vapor 4 is a point source, the stream of vapor 4 has a cos .sup.n .alpha. distribution. Therefore, if such a technique is combined with an oblique incidence vacuum evaporation process, the problems will occur in that the efficiency, with which a thin film of the deposition material is formed, cannot be kept high. Specifically, as shown in FIG. 14, a substrate 10 is fed from a feed shaft 11, conveyed along the circumferential surface of a cooling can 13, and wound around a wind-up shaft 12. Also, a deposition material contained in a crucible 15 is evaporated in a vacuum chamber, and the resulting stream of vapor 16 impinges upon the substrate 10. In this manner, a thin film of the deposition material is formed on the substrate 10. At this time, the angle of incidence of the stream of vapor 16 upon the substrate 10 is limited by a shield plate 14. Specifically, only part 16a of the stream of vapor 16, which part is hatched in FIG. 14, impinges upon the substrate 10, and the other part of the stream of vapor 16 impinges upon inner wall surfaces of the vacuum chamber, or the like. Therefore, with the conventional process, only part of the stream of vapor 16 is utilized to form a thin film of the deposition material, and the efficiency, with which a thin film of the deposition material is formed, cannot be kept high (e.g. the efficiency is at most 10%). Accordingly, the efficiency, with which the deposition material is utilized, is low, and the productivity cannot be kept high.